A crossover is a fanfic in which two or more fandoms are combined in some way. Crossovers are an old story trope of fanfiction; as soon as fans started writing stories in more fandoms than just Star Trek, they started crossing those sources together. In story notes, a crossover is frequently notated with an 'x', so a Star Trek story crossed over with Star Wars is sometimes written as ST/SW and sometimes written ST x SW.

Crossovers may take place in many ways. In the most common form, characters from Fandom A may meet characters from Fandom B (i.e., a Stargate Atlantis story in which The Doctor visits Atlantis, perhaps with a Companion or two in tow). Crossovers can involve as many fandoms as the author(s) can juggle; there's no limit to the potential madness and creativity of the exercise.

A form that is most common in Harry Potter crossovers, but sometimes seen in other fandoms with canon orphans (or characters who can easily be made to be orphans), is the adoption fic, in which someone from another fandom (generally selected for their kick-ass qualities) brings up the orphan.

Many fandoms use canon incidents or devices to facilitate crossovers. Star Trek's canon holodeck can be used to introduce characters from any other fandom, and its time travel allows crossovers with present-day or historical settings. In Buffy fandom the first Halloween episode involved enchanted costumes which made the wearer take on the personality of the person depicted; in the episode these were generic characters - soldiers, ghosts, etc. - and the effects wore off at the end of the spell, but in fanfic the costumes are often used to give Buffyverse characters the personality and sometimes powers of e.g. a superhero, often permanently. This trope has been borrowed by many other fandoms. Doctor Who introduced the "chameleon arch" which turns Time Lords into normal humans with false memories, revealed only when they open a device that contains their memories; this is often used in fanfic to turn characters from other fandoms into Time Lords.

Some crossovers are almost subliminal, like cinematic Easter eggs, briefly using e.g. a character or device from one canon in a story set in another, which may go almost unnoticed. They are occasionally seen in canon as well as fan works. For example, The Laughing Fish, an episode of Batman: The Animated Series, has a scene in which Joker tries to kill Batman with a wrench he grabs from a box that appears to have a partially-obscured Binford Tool label, a Home Improvement reference. Buzz Lightyear is imprisoned in a box with the Binford logo in Toy Story. Similar minor crossovers are common in fan works.

Fusions

Fusions are a kind of crossover or AU where characters from one fandom (Fandom A) may be imported into the setting of another fandom (Fandom B). Fandom A characters may take on the roles of Fandom B characters (i.e., a Stargate Atlantis story in which John Sheppard is a Sith lord and Rodney McKay is a Jedi) (perhaps in a story strangely like this one: The Dark Side, by Astolat) and not interact with Fandom B characters at all, or only very peripherally.

Crossover fusions with romance movies (An Affair to Remember, American Dreamer, You've Got Mail, etc.) have been written in slash forever, but for some reason, they're rarely considered crossovers; they're just considered AUs, perhaps because the romance movies rarely have a fandom following of their own.

Actor crossovers

A special case are crossovers between sources that share the same actor, e.g. between SGA and Traders which both star David Hewlett. These same-actor crossovers sometimes draw criticism for requiring more suspension of disbelief, and in the case of same actor pairings also for blurring the line between character and actor slash, especially in the past when RPS was less widely accepted.[1] In some cases these crossovers even rate their own pairing names, communities and challenges.[2] For instance, in SGA fandom, a common crossover technique is to write a character of David Hewlett's (from a show other than SGA) in a romance with a character of Joe Flanigan's (also from a show other than SGA) - this crossover pairing is known as Hewligan, and can refer to a variety of pairings (i.e., Grant Jansky (Traders)/Pete Sherman (Sherman's March), or Michael (Century Hotel)/Nick (Farewell to Harry).[3]

In other cases, two characters played by the same actor turn out to be the same person; for example Claude Rains from Heroes turns out to be the Ninth Doctor in disguise, or Ducky Mallard from NCIS is actually an older Illya Kuryakin from Man from Uncle. See also the Six Degrees of Jeremy Renner trope, in which all characters played by Jeremy Renner (Hawkeye in The Avengers) are Clint Barton in disguise, siblings, clones, etc. A canon non-crossover example is the case of the vengeance demon Halfrek in Buffy the Vampire Slayer, played by Kali Rocha who had previously played William's (pre-vampiric Spike's) love-interest Cecily. In a later episode it was revealed that Halfrek was Cecily; comics canon says she was already a demon when William was in love with her.

Stories in which characters played by the same actor are mistaken for each other, or impersonate each other, are also common. For example, Nathan Fillion played the murderous preacher Caleb in Buffy, and author Richard Castle in Castle; a standard trope seen in most crossovers between the shows has Buffy characters attack Castle as soon as they see him, assuming that Caleb has somehow come back to life. In non-crossover canon much of the plot of Orphan Black revolves around repeated cases of same-actress impersonation or mistaken identity.

Same name crossovers

A common if somewhat unimaginative trope takes two fandoms which happen to have characters with the same name. "Obviously" they're related! An example is Scott Summers and Buffy Summers, and there are several stories where they are cousins or otherwise related. This can be stretched slightly by ignoring differences in the spelling of the surname - e.g. Jamie Somers (The Bionic Woman) is also related to Buffy, but a different branch of the family that changed the name slightly. There are stories where Samantha Carter (Stargate series) is related to Agent Carter (Marvel) or Sheriff Carter (Eureka). It's even occasionally extended to fandoms where characters have common first names, if the name is rare enough; e.g. Xander Cage (the XXX series) and Xander Harris (Buffy).

An alternative approach is to treat common names as a coincidence which results in the events of the crossover; e.g., Buffy and Scott happen to reserve rooms at the same hotel and the desk clerk somehow thinks they're married and ends up putting them into the same room; or a message intended for the Watcher Bernard Crowley (Buffy) ends up going to the fallen angel Crowley (Good Omens). This approach is comparatively rare.

Location Crossovers

Another way of combining sources is to build on their use of common locations. For example, The Avengers Movieverse has probably been crossed with every TV show or film that has a contemporary New York setting. CSI Miami has been crossed with Burn Notice, Dexter, and other shows with a Florida setting. Another example is Wholock.

Using official crossovers

In some fandoms, it's not unusual for two or more fannish sources to have crossover episodes or events in canon. Typical examples are comic company crossovers between DC and Marvel, like the JLA vs. Avengers mini-series, and crossovers between Batman and Judge Dredd, Predator, etc. TV shows (especially ones made by the same production company) also do crossovers, such as those between Eureka and Warehouse 13. The latter can be double episodes like between CSI: Las Vegas and Without A Trace (or back in the day, between Magnum PI and Simon & Simon) or implied crossovers through character appearances, like of Homicide's John Munch showing up, in character, on the X-Files. An unusual example was the X-Files episode X-Cops which was a crossover with the "reality" crime show Cops, though in this case using actors instead of real police. In some TV series the official crossover implications are almost byzantine through a series of such overlaps, and tracked on fansites devoted to this phenomenon.[4]Doctor Who and Torchwood are so intertwined that many authors and sites consider them to be a single fandom; The Sarah Jane Adventures also derive from the same canon, and stories that mingle canon from all three shows plus spinoff novels etc. are moderately common. The recent spin-offClass (2016) will probably be incorporated into this common fandom too. Arrow and its spin-offs The Flash, Legends of Tomorrow, and Vixen are often entwined with each other, fans often nicknamed the universe Arrowverse, Flarrow, and Legends of Flarrow. When NBC cancelled Constantine, it allowed the CW to have an official crossover between the two and thus the others. The official crossover episode between The Flash and the CBSSupergirl pleased many fans as did Supergirl's move to the CW; which officially brought more news of more crossovers between all four shows.

Some crossovers are motivated by simply trying to find universes that couldn't possibly by crossed over, and then pulling it off. Or not. Purists only cross universes that could possibly exist in each other's reality, and have more or less the same tone. Non-purists admit to no limits.

Ofttimes crossovers are an answer to the problem of a much-loved character who lacks a shippable partner in his or her source fandom, or else an answer for a writer who only likes one character of a popular pairing -- like Methos, but not Duncan? Write Methos into another universe altogether! Certain characters seem to lend themselves to showing up in other universes; they are sometimes referred to as Little Black Dresses of fandom.

Fan Reaction

[1990]:
Writing a crossover has its own dangers, the difficulty lying somewhere between single-media and professional writing. I like to write them because they add a bit of spice to a story and provide a challenge. I rarely enjoy reading them, however, because they're often poorly done; the writer or editor can't handle one of the shows involved or doesn't know enough about it, one of the shows is sacrificed for the glorification of another, humorous crosses aren't half as funny to the reader as they seem to the writer and editor, etc. Barring copyright problems, even television has problems with crossovers (does anyone remember the Magnum, P.I. and Murder, She Wrote crossover, where Magnum looked like a raving idiot and Jessica Fletcher came off as a meddlesome old hag?). Crossover fiction can be written and written well, no matter how outrageous the crossover. [7]

The Kingdom of the Blaggers by Hackthis is a crackfic fusion: "The problem with being the son of Uther Pendragon, legend of stage and screen, winner of Emmys (twice), Tonys (once), Oliviers (multiple), BAFTAs (twice), Oscars (once) and People's Sexiest Man (twice) was that nobody was ever going to take Arthur seriously as an actor in his own right." (2009)

Sherlock, Elementary: Elementary, My Dear Sherlock by Eltuine. "Joan Watson wakes up in an unfamiliar house in an unfamiliar city, with an unfamiliar man staring at her, claiming to be Sherlock Holmes. Meanwhile, on the other side of the ocean, John Watson finds himself in a similar situation. When each Watson tries to return home, however, difficulties arise. How will both Sherlocks deal with the removal of their respective Watsons, and how will each Watson get back to where they belong?" (2013- WIP)

Star Trek: The Next Generation, Buffy: Legend by Shayne Terry. "Trapped by an alien collector, Lieutenant Commander Data discovers an early 21st century android excavated from the ruins of Sunnydale and a capsule containing the frozen body of one of the Eugenic wars most notorious figures...Buffy Summers." (2011)

Mr. Benn, The Invisibles, RPF: Mr. Benn: Invisible by dontmakeeyecontact. When Jack Frost and Jolly Roger find they need to break into a record company’s impregnable headquarters they call upon the help of a singular individual. Crossing a popular British children's cartoon series of the 1970s with Vertigo's The Invisibles and rock RPF. (Date unknown)

Doctor Who, Nero Wolfe: The Orchid Thief by leviathan: Nero Wolfe and Archie Goodwin have solved a lot of tough cases in their careers as detectives. But never one like the one that begins with the new arrival in the Plant Rooms. Before it ends, they'll deal with a tattooed lady, miniaturized corpses, and Things from Another World! (2009)